Solid state gamma and X-ray detectors are used for many applications which require precise spectroscopic measurements. These applications include security, medical, space and astrophysical research, reactor safety, and host of others. A gamma or X-ray interacting with a solid state detector produces secondary ionizing radiation which create electron-hole pairs. The number of electron-hole pairs produced is directly proportional to the energy of the absorbed gamma or X-ray. Under the influence of the electric field existing between the electrodes of the detector the electrons and holes drift towards the positive and negative electrodes, respectively, where they are collected. The drifting electrons and holes induce signals on the electrodes which are amplified. The induced are proportional to the energy of the absorbed gamma or X-ray, and thus good spectroscopic measurements are obtained. Solid state detectors and their associated electronics tend to be compact, require little power, and their stabilization time is small.
For very accurate spectroscopic measurements, germanium detectors (i.e. Ge(Li) ) are used (see, for example, F. S. Goulding, Nuclear Instruments and Methods, Vol. 43, pp. 1-54, 1966). These detectors provide very accurate energy measurements because of the low energy required to produce an electron-hole pair, and the correspondingly large number of electron-hole pairs produced per gamma or X-ray interaction. Germanium detectors must be operated at liquid nitrogen temperatures because of the very small energy band gap. Since they operate at liquid nitrogen temperatures, mobilities are high and charge collection efficiencies are effectively unity for both electrons and holes. Imaging systems with germanium detectors exist, but are costly because the whole imaging system needs to be at liquid nitrogen temperature.
Gamma and X-ray detectors and imaging system have also been made from silicon. For gamma or X-rays with energy above 20 keV, silicon suffers from its low photoelectric absorption probability, due to its low atomic number, Z=14. Nonetheless, silicon detectors are very useful for gamma and X-rays with energies less than 20 keV. Recently, imaging system prototypes have been made from silicon with readout segmented onto strips or pads of 50-100 .mu.m spacing (see, for example, A. Czermak, et al., Nuclear Instruments and Methods, Vol. A360, pp. 290-296, 1995.). Spatial resolutions of several .mu.m have been achieved by determining the centroid of collected charge distribution on the strips or pads. The physical effect which distributes the collected charge (electrons or holes) among several strips or pads is the diffusion of the charges as they drift in the electric field towards the electrodes. The transverse diffusion of the charge carriers typically expands over an area of about 50-100 .mu.m.
Much effort has gone to developing room temperature solid state detectors with medium to high atomic number. Some of the materials which have shown promise are: CdTe, CdZnTe, Hgl2GaAs, PbI2 (M. Cuzin Nuclear Instruments and Methods, Vol. A253, pp. 407-417, 1987; Y. Elisen, Workshop on Solid State Gamma and X-ray Detectors, Grenoble, September 1995, to be published in Nuclear Instruments and Methods). These materials have a high absorption probability, even for gamma rays with energies above 100 keV. Room temperature operation is a very important consideration in many applications. However, these materials suffer from bad to poor charge transport properties for the holes. As a result, detectors from these materials exhibit incomplete charge collection properties, whereby only a fraction of the photoelectric conversions appears in a distinct photopeak, and the rest of the events show up in a broad "incomplete" low energy region. To correct for this many techniques, both in hardware and software, have been developed. One of the more popular schemes, especially for CdTe detectors, involves correlating lower charge collection with longer risetimes of the pulses. The longer risetimes indicates a deeper interaction of the gamma or X-ray in the crystal which requires a larger fraction of the collected charge to be induced by the holes (see, for example, Y. Eisen and Y. Horowitz, Nuclear Instruments and Methods, Vol. A353, pp. 60-66, 1994).
A lot of emphasis has been placed on developing imaging systems utilizing room temperature solid state detectors. Imaging systems have been constructed with an array of individual detector elements, where each detector element forms a pixel of the imaging systems. Monolithic solid state detectors with segmented readout, usually with pad segmentation, with each pad serving as a pixel in the imaging system, have also been developed. Segmentation of the readout in monolithic detectors is convenient and economic in that it saves a lot of processing and machining of the detector material during production.
More recently, attention has been focused on CdZnTe as a promising material for room temperature solid state gamma and X-ray detectors (see, for example, J. F. Butler, C. L. Lingren, and F. P. Doty, IEEE Trans. Nuclear Science, Vol. 39, No. 4, pp. 605-609, 1992). CdZnTe has relatively high mean atomic number (Z-50 as compared Z=32 for germanium). It also has very high resistivity, p.apprxeq.10.sup.11 .OMEGA.-cm, and as a result very low leakage current. Low leakage current means very little noise, and insensitivity to changes in temperature (i.e. dark current).
CdZnTe detectors, despite exhibiting excellent charge transport properties for electrons, show fair to poor charge transport properties for holes. As a result techniques for obtaining good spectroscopy using only the induced signals from the electrons have been developed, irrespective of the holes or depth of interaction. A scheme employing parallel grids at the anode, which is sensitive to the electron signal only has been developed (P. Luke, Applied Phys.Lett., Vol. 65, pp. 2884-2886, 1994). This scheme requires fine segmentation of the positive electrode into strips with a small difference in bias between alternating strips. The electrons are collected on the strips with the slightly higher bias.
An approach using small segmented readout elements at the anode has also been developed (H. H. Barrett, J. D. Eskin, and H. B. Barber, Phys.Rev,Lett., Vol. 75, pp. 156-159, 1995). In this approach, the smaller the readout element size the less the sensitivity to incomplete charge collection of the holes, since the small readout element would only feel induced charge from electrons drifting in close vicinity to the readout element. This approach obviously also benefits from the very fine spatial resolution accorded by the small readout elements, although at the cost of large number of channels of electronics and also the corresponding slowness in readout acquisition time.
Solid state detectors hold the potential of very fine spatial resolution, and therefore very good imaging capabilities. The compactness of solid state detectors and the readout electronics enable fabrication of small detector elements, where each detector element in an array of detectors can comprise a pixel of the image formed. Imaging systems have been constructed with individual solid state detector elements of sizes as small as 1 mm. A more economical solution, which avoids individual processing of each detector element at the manufacturing stage, is to fabricate monolithic solid state detectors with segmented readouts, where either the positive or negative electrode is segmented in any desired fashion dictated by the readout requirements. Each detector readout element has its own charge sensitive electronics. Readout electrodes in monolithic solid state detectors are typically segmented into pad (i.e., square) elements or strip elements, although other segmentation geometries can be applied. Usually, in a pad segmentation configuration, the position of the incident X-ray or gamma ray in the image is obtained from the planar location of the pad which received the signal within the array. In such a case, the spatial resolution of the image is limited to the size of the pad. Smaller pad elements can therefore lead to better spatial resolution, but with an quadratic increase in detector segmentation and a correspondingly quadratic increase in cost of readout electronics and readout complexity. Readout from segmented electrodes, especially in silicon detectors, has also been configured in strip geometry. Strip geometry enables a finer spatial resolution with only a linear increase in segmentation (and therefore price of electronics), but at the cost of requiring projective readout with the ambiguities and slow readout inherent in projective readout. Additional shortcomings of strip readout include inability to handle high counting rates, and growth of noise with increased detector area. Simultaneous x and y measurement can be accomplished by using double sided strip readout, with the strips on one side of the detector perpendicular to the strips on the other side.
It is an object of the present invention to provide a method for obtaining image spatial resolution smaller than the readout element size (i.e., sub-pixel resolution) of a segmented room temperature solid state detector, thereby affording improved image resolution without incurring the costs involved in using smaller readout elements.
It is a further object of the present invention to provide a method for correcting the incomplete charge collection and improving the energy resolution of a room temperature solid state detector, provided the detector has segmented readout. This will lead to much better spectroscopic performance of the detector. An imaging system comprising of superior spectroscopic detectors will provide for improved image contrast.